Many computer systems execute background processes. The tasks of these processes are often directed to performing maintenance (housekeeping) functions, such as file compression, disk defragmentation, file-system content indexing and file archiving. While these tasks are relatively important to complete at some time, there is ordinarily no need to do so at any particular time. Thus, they are often run in the background, operating when no higher-priority foreground process, such as a process of an application program interacting with a user, is operating.
The traditional approach to operating a background process is to assign the process a priority level that is just above the system idle process, whereby CPU (central processing unit) cycles are allotted to the background process only when no normal priority process is ready to use the CPU. For example, in the Unix programming environment, the command “nice” initiates a process with reduced scheduling priority, which may be set as low as the system idle priority level.
Governing the execution of a process by priority scheduling of CPU cycles assumes that the CPU is the limiting resource. However, in many instances, the CPU is not the limiting resource, but instead, process performance may be limited by I/O (input/output) rate and/or contention for another system resource, such as the disk, processor cache, memory capacity and/or memory bandwidth. Since background processes contend for these resources, background processes interfere with higher priority foreground processes, and scheduling priority is insufficient as a mechanism for limiting this interference. The interference increases with background processes related to file-system maintenance activities, since such activities are resource intensive, but often use few CPU cycles.
By way of example, consider a background process that wants to access the same I/O resource (e.g., a disk) as a foreground process, wherein both processes are regularly performing I/O operations to the disk resource that take longer than the amount of time each process needs the CPU. When the foreground process is performing disk I/O operations, the background process is given some CPU cycles, during which time the background process desires access to the disk resource. As can be appreciated, the background process has to wait for completion of the I/O by the foreground process.
When the foreground process completes its I/O operation, the background process is given access to the disk and the foreground-process is given CPU cycles, during which time the foreground process again wants access to the disk resource. However, because the foreground process has to wait for access to the disk resource, the foreground process has to idle, and at that moment, the CPU is essentially not being used by either the foreground or background process. Accordingly, the CPU is not the limiting resource, but rather the disk is, i.e., contention for the disk resource by the background process causes the interference with the foreground process. Unless the background process is entirely halted, however, the background process will receive CPU cycles when the foreground process is performing I/O, and thus will continue to interfere with the foreground process because the background process also will request access to the disk resource during its allotted cycles.
A foreground application program's process that is of immediate consequence to a user is thus negatively impacted by (non-CPU) resource contention, unless the background process is canceled. However, simply canceling the background process defeats the many advantages of having a background process run, e.g., to perform useful work during the many times when the foreground process is not contending with the background process for the CPU or other resource.